The invention relates to a process for the simultaneous obtaining of a tert. amyl ether (particularly TAME)-rich fraction and a n-pentane-rich fraction from a C.sub.5 fraction containing isopentenes, cyclopentene and cyclopentadiene.
Cracking processes such as steam cracking, viscoreduction, coking and catalytic cracking supply olefin-rich C.sub.5 fractions. Certain of these can contain significant proportions of methyl butenes (isopentenes).
This is in particular the case with steam cracking C.sub.5 fractions, which can contain up to 10% of the mixture 2-methyl-1-butene, 2-methyl-2-butene and 3-methyl-1-butene. This fraction can simultaneously contain up to 20% of diolefins in the form of isoprene, pentadiene and cyclopentadiene. A typical composition of this fraction is given in table 1.
TABLE 1 ______________________________________ % by weight ______________________________________ C.sub.4.sup.- 1 nC.sub.5 26 isoC.sub.5 24 nC.sub.5.sup.= 4.5 Methyl butenes 12.0 Cyclopentene 1.5 Isoprene 13.5 Pentadiene 9.0 Cyclopentadiene 7.5 C.sub.6.sup.+ 1.0 ______________________________________
On the basis of a catalytic cracking C.sub.5 fraction, it is possible to hydrogenate the diolefins into olefins (with the exception of methyl butenes) as is described in the assignee's U.S. Pat. No. 4,724,274. The reaction takes place by passing the charge to be treated (cracking C.sub.5 fraction) with hydrogen and 2 to 50 ppm by weight (expressed as sulphur based on the charge) of at least one compound of sulphur into contact with a supported catalyst containing at least one noble metal of group VIII, at a temperature of 20.degree. to 150.degree. C. and a pressure of 5 to 100 bar.
Moreover, during said hydrogenation stage it is possible with respect to the isopentene to isomerize 3-methyl-1-butene and 2-methyl-1-butene into 2-methyl-2-butene in order to obtain a distribution of these products in proportions close to thermodynamic equilibrium. This composition at equilibrium is shown in table 3.
TABLE 3 ______________________________________ % by weight ______________________________________ 3-methyl-1-butene 0.5 2-methyl-1-butene 12.5 2-methyl-2-butene 87.0 ______________________________________
The steam cracking C.sub.5 fractions differ from the catalytic cracking C.sub.5 fractions by the presence of cyclopentadiene and cyclopentene in a significant quantity (cf. table 1). It has surprisingly been found that the use of a steam cracking C.sub.5 fraction in place of a catalytic cracking C.sub.5 fraction makes it possible to obtain a distribution of isopentenes even closer to thermodynamic equilibrium contitions.
It is also known that fractions rich in methyl butene (isopentenes) are preferred charges for the etherification of iso-olefins having 5 carbon atoms and which are also called isoamylenes by an alcohol (e.g. methanol) for producing a tert. amyl alkyl ether (e.g. tert. amyl methyl ether or TAME). This ether can advantageously be used in mixed form in car fuels in order to improve their research octane number (R.O.N.) and motor octane number (M.O.N.). These etherification processes are described in numerous patents, e.g. U.S. Pat. No. 4,336,407.
In general terms, prior to the etherification stage, the olefin compounds are not separated from the paraffin compounds in particular due to the high separation costs. After etherification the product obtained is a mixture of paraffins, olefins and tert. amyl alkyl ether (e.g. TAME).
It should be noted that only the iso-olefins present in the charge to be etherified are converted into ether. Thus, straight and cyclic olefins and saturated molecules are refractory to the etherification reaction.
The mixture obtained can be used directly in the composition of a car fuel, bearing in mind its properties. However, this has the disadvantage of introducing olefin compounds into the final fuel. It is known that although olefins generally have relatively high research octane numbers (R.O.N.), their motor octane numbers (M.O.N.) are very low. The development and marketing of increasingly high performance and sophisticated engines, as well as the elimination of lead from fuel, made necessary by the introduction of catalytic converters, have led to increasingly severe octane number and in particular M.O.N. specifications for car fuels. The constraints associated with the protection of the environment make it necessary to reduce the olefin content of fuels. Thus, it becomes disadvantageous to introduce these residual olefins into the fuels. This evolution has led to the appearance of separating columns downstream of the etherification units, so as to separate the olefins and paraffins from the ether produced. At present the standard practice consists of using ether for the fuels and recycling the mixture of paraffins and olefins to the steam cracking unit so as to produce other valorizable compounds such as ethylene and propylene. However, it is known that the ethylene and propylene yields obtained during steam cracking are very highly dependent on the quality of the charge to be cracked.
It is generally accepted that the ethylene and propylene yields are much higher when the cracked charge is formed from saturated molecules such as isoparaffins or preferably normal paraffins.